1. Technical Field
The present invention relates to a multi-nozzle burner for manufacturing a porous glass base material, which includes a small diameter auxiliary gas ejection port in a burnable gas ejection port, and to a manufacturing method of a porous glass base material. The present application claims priority from a Japanese Patent Application No. 2009-40775 filed on Feb. 24, 2009, the contents of which are incorporated herein by reference.
2. Related Art
Conventionally, various methods have been proposed to manufacture an optical fiber base material. Among these methods, outside vapor phase deposition (OVD) is frequently used because comparatively various refractive index distributions can result and optical fiber base materials of a large diameter can be mass produced. In OVD, a starting member of about 50 mm φ is rotated and either the burner or the starting member is reciprocated relative to each other, then glass particles generated in the burner flame are deposited to generate a soot having a thickness of about 250-350 mmφ, and the result is dehydrated and sintered in an electric furnace.
A concentric multi-tube burner has been conventionally used to synthesize glass particle depositions. A burner having such a structure cannot sufficiently mix the glass raw material gas, the burnable gas, and the auxiliary gas, which has resulted in insufficient generation of glass particles, to lead to poor yields to prevent high speed synthesis.
So as to solve this problem, Patent Document No. 1 proposes a multi-nozzle burner having a burnable gas ejection port, in which a small diameter auxiliary gas ejection port is provided to be around and centered on a raw material gas ejection port. Patent Document No. 2 proposes reducing the material thickness of the small diameter gas ejection port, and a method of increasing the speed of ejected gas flow to 20 m/s or faster. Patent Document No. 3 proposes making the edges of the dividing walls between gas flow paths of a multi-tube burner to have acute angles, blunt angles, or round shapes. The following lists the patent documents used in this specification.    Patent Document No. 1: Japanese Patent Publication No. 1773359    Patent Document No. 2: Japanese Patent Application Publication No. H6-247722    Patent Document No. 3: Japanese Patent Application Publication No. H11-21136